1. Field of the Invention
The invention relates in general to a front-end for a digital video receiver and a method for automatic gain control thereof, and more particularly to a front-end for a digital video receiver and a method for automatic gain control thereof that may save time.
2. Description of the Related Art
Usually, in the applications of the wireless communication technology, when the power level of the received radio frequency (RF) signal changes, the output of the wireless communication system also changes therewith. In order to obtain the best output, a method for automatic gain control (AGC) is utilized for tuning.
FIG. 1 (Prior Art) is a schematic illustration showing a front-end 100 of a conventional communication system. Referring to FIG. 1, the front-end 100 includes a tuner 102, an intermediate frequency (IF) amplifier 104, a demodulator 106 and a low pass filter 108. The tuner 102 receives a RF signal RF and converts the RF signal RF to an IF signal IF. The IF amplifier 104 has a gain, and the IF amplifier 104 amplifies the IF signal IF according to the gain.
The demodulator 106 demodulates the amplified IF signal IF into a base-band signal, and encodes the base-band signal to a transport stream TS. Besides, the demodulator 106 outputs a pulse width modulation (PWM) signal PWM, which is related to the gain of the IF amplifier 104. The demodulator 110 has an internal automatic gain target AGC_target.
The low pass filter 108 includes a capacitor C and resistors R1 to R4, as shown in FIG. 1, wherein the resistors R2 and R4 are variable resistors. The low pass filter 108 passes a DC level of the PWM signal PWM to be an automatic gain control signal AGCS, and feeds the automatic gain control signal AGCS back to the IF amplifier 104 to control the gain of the IF amplifier 104, and thus to change the transport stream TS to obtain a better display image frame.
Usually, the automatic gain target AGC_target of the demodulator 106, the resistor R2 and the resistor R4 of the low pass filter 108 are changed to control the gain of the IF amplifier 104. However, during the tuning process, it is time-consuming and complicated to try many combinations of the automatic gain target AGC_target, the resistor R2 and the resistor R4. In addition, when the received RF signal RF is a small signal, the combination of the automatic gain target AGC_target, the resistor R2 and the resistor R4 capable of enabling the better display image frame is also different from that in the condition when the received RF signal RF is a large signal.